Space Launch System, a NASA rocket designed by Boeing that will be intended for deep-space exploration to near-Earth asteroids, the moon and even Mars, is depicted in this rendering. The first flight test of the $2.99-billion rocket is scheduled for 2017. Photo: NASA

Boeing, prime contractor on the rocket, announced on Wednesday that it had completed a critical design review and finalised a $US2.8-billion ($2.99-billion) contract with NASA. The last time the space agency made such an assessment of a deep-space rocket was the mighty Saturn V, which took astronauts to the moon.

If all goes well, the rocket's initial test flight from Cape Canaveral, Florida, is expected in 2017.

"We're ready to move forward," says Frank McCall, Boeing's Space Launch System deputy program manager. "This program has the potential to be inspiring for generations." Space Launch System at liftoff is depicted in this rendering. Photo: NASA

"We're ready to move forward," said Frank McCall, Boeing's Space Launch System deputy program manager. "This program has the potential to be inspiring for generations."

The Space Launch System has been the subject of criticism that its goals and timeline are too vague. It also faces additional funding questions from Congress in the years ahead.

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"We're not operating on the budgets of Apollo missions anymore," McCall said. "But we're not operating on a shoestring budget either."

Reaching this milestone has been four years in the making. In 2010, President Obama laid out a new vision for the nation's space ambitions, focusing on future deep-space missions and scrapping a manned moon mission called Constellation.

Space Launch System's design called for the integration of existing hardware, spurring criticism that it's a "Frankenstein rocket," with much of it assembled from already developed technology. For instance, its two rocket boosters are advanced versions of the Space Shuttle boosters, and a cryogenic propulsion stage is based on the motor of a rocket often used by the Air Force.

The Space Frontier Foundation, an advocacy group and frequent NASA critic, said Space Launch System was "built from rotting remnants of left over congressional pork. And its budgetary footprints will stamp out all the missions it is supposed to carry, kill our astronaut program and destroy science and technology projects throughout NASA."

Currently NASA has no way to get its astronauts to the International Space Station other than paying $US71 million ($75.81 million) to Russia for a ride. NASA ultimately wants private companies to take astronauts to the station, but that hardware isn't yet ready.

Instead, the space agency wants to focus its attention on deep-space missions aboard Space Launch System, including a mission to land on an asteroid by the mid-2020s.

But that plan has failed to gain widespread support, reflecting serious concerns about the billions of federal dollars needed and a lack of detail about the most difficult aspects of the mission. The total cost of the program and which asteroid NASA would visit remain unknown.

The Government Accountability Office said in a study of the program that funding remains a top risk. NASA plans to spend about $6.8 billion to develop the rocket in fiscal years 2014 through 2018.

Boeing says the advantage of building Space Launch System is that it can carry out a "menu of missions" that include shooting astronauts to the moon and Mars, in addition to far-flung asteroids.

The Planetary Society in Pasadena, another space advocacy group, initially came out against the plan to build the rocket because it lacked one specific mission. Now the group says Space Launch System will play an important role because of its versatility.

"It has a lot of potential not only for human missions but robotic missions as well," said Casey Dreier, the group's director of advocacy.

Work on the 98-metre Space Launch System is spread throughout Southern California, including Boeing's avionics team in Huntington Beach. The rocket's core stage will get its power from four modified space shuttle main engines built by Aerojet Rocketdyne in Canoga Park.

There are two versions of the rocket being designed. One will carry up to 69,853 kilograms and a later version will carry up to 129,727 kilograms.

The rocket is also being designed to carry the capsule-shaped spacecraft Orion, which is built by Lockheed Martin. It can lug up to four astronauts beyond low-Earth orbit on long-duration missions.

The first Space Launch System mission in 2017 will launch an empty Orion spacecraft. The second mission is targeted for 2021 and will launch Orion and a crew of NASA astronauts.